Coel (Old Welsh: Coil), also called Coel Hen (Coel the Old) and King Cole, is a figure prominent in Welsh literature and legend since the Middle Ages. Early Welsh tradition knew of a Coel Hen, a c. 4th-century leader in Roman or Sub-Roman Britain and the progenitor of several kingly lines in Yr Hen Ogledd (the Old North), a region of the Brittonic-speaking area of what is now northern England and southern Scotland.

Later medieval legend told of a Coel, apparently derived from Coel Hen. He was said to be the father of Saint Helena of Constantinople and through her the grandfather of Roman Emperor Constantine the Great. Although it is likely to be erroneously identifying Saint Helen of Caernarfon.

Other similarly named characters may be confused or conflated with the Welsh Coel. The legendary "King Coel" is sometimes supposed to be the historical basis for the popular nursery rhyme "Old King Cole", but this has been said to be unlikely.

Coel's name was rendered "Coil" in Old Welsh. Rare or unique as a Welsh name, its origin has long been seen as uncertain. John T. Koch has argued that it is simply the common noun which in Modern Welsh has the form coel, meaning "belief, credence; confidence, reliance, trust, faith" (and the secondary meaning "omen"), derived from Proto-Celtic *kaylo- "omen" and ultimately from Proto-Indo-European *keh₂ilo- "whole, healthy; blessed with good omen". In Koch's view, "these semantic and etymological aspects allow that Coil could be a legendary, or even a mythological, founder" of the dynasties that bear his name rather than a historical figure.

It may be the same as the Coel is often named as "Coel Hen", Hen being an epithet meaning "old" (i.e., "Coel the Old"). The genealogies give him an additional epithet or patronym, Godebog (Old Welsh: Guotepauc), meaning "Protector" or "Shelterer". His name is thus sometimes given as "Coel Godebog" or "Coel Hen Godebog". However, some of the Harleian genealogies list Godebog as Coel's father's name. Geoffrey of Monmouth rendered the name as both Coel and Coillus in his Historia Regum Britanniae. Some modern authors render it as "Cole".

The historian Ben Guy has commented that "Coel Hen's role in medieval texts is solely legendary; there is no merit whatsoever in any attempt to assign him some historically significant role in late Roman Britain, as many over the years [...] have fruitlessly attempted to do". Coel Hen appears in the Harleian genealogies and the later pedigrees known as the Bonedd Gwŷr y Gogledd (The Descent of the Men of the North) at the progenitor of several post-Roman royal families of the Hen Ogledd. His line, collectively called the Coeling, included such noted figures as Urien, king of Rheged; Gwallog, perhaps king of Elmet; the brothers Gwrgi and Peredur; and Clydno Eiddin, king of Eidyn or Edinburgh. The poem Y Gododdin mentions some enmity between the "Sons of Godebog", possibly a reference to the Coiling, and the heroes who fought for the Gododdin at the Battle of Catraeth.

As an ancestor figure, he compares to Dyfnwal Hen, who is likewise attributed with founding kingly lines in the Hen Ogledd. Hector Boece and Ayrshire folklore both state that Coel and his entire army perished in the Battle of Coilsfield. According to Welsh tradition the region of Kyle was named for Coel, and a mound at Coylton in Ayrshire was regarded as his tomb. Projections back from dated individuals suggest that Coel Hen would have lived around AD 350–420, during the time of the Roman departure from Britain. In his book The Age of Arthur, historian John Morris suggested Coel may have been the last of the Roman Duces Brittanniarum (Dukes of the Britons) who commanded the Roman army in northern Britain, and split his lands among his heirs after his death. However, Morris's book has been widely criticized. It has been suggested that Coel was appointed governor of northern Britain, ruling from Eburacum (York), by Magnus Maximus.

In the twelfth century, a story arose claiming that Colchester in Essex was named after a man called Coel, who was the father of Saint Helena, and therefore the grandfather of Constantine the Great. Though not initially associated with Coel Hen, the two Coels began to be conflated in Welsh scholarship from the fifteenth century. The legend originated from a folk etymology indicating that Colchester was named for Coel (supposedly from "Coel" and "castrum", producing "fortress of Coel"). However, the city was actually known as Colneceaster until the n was dropped in around the 10th century; its name likely comes from the local River Colne.